Cohesive properties and electronic structure of HeuslerL21-phase compoundsNi2XAl (X=Ti, V, Zr, Nb, Hf, and Ta)

The ternary (Heusler) L$2_1$-phase compounds ${\mathrm{Ni}}_2$XAl (group-IVA or -VA element X=Ti, V, Zr, Nb, Hf, and Ta) have been found to greatly increase the high-temperature creep strength of NiAl. We report here the cohesive properties and electronic structures of these potential high-temperature structural materials in both the Heusler L$2_1$ and the B2 phases determined by means of the all-electron total-energy self-consistent linear muffin-tin orbital method based on the local-density approximation. Our results show that the calculated equilibrium lattice constants of the L$2_1$-structure compounds are in very good agreement with experiment. For ${\mathrm{Ni}}_2$TiAl and ${\mathrm{Ni}}_2$VAl, the lattice constants are found to match that of NiAl (mismatch is 1.7% and 1.0%, respectively). But for the other four compounds, the mismatch is found to be larger (3.6–5.6%). The difference of the lattice constants in the B2 and the L$2_1$ structures, however, is very small (less than 1%). The formation energy is found to be consistently in favor of the L$2_1$ phase. The Ni d and X d hybridization contributes the most to the cohesion of these compounds whose stability is also made certain by the well-separated filled bonding and empty antibonding states in the density of states. The density of states of the B2 phase is quite similar to that of L$2_1$, but distinctions exist due to their structural differences. The rigid-band approximation is found to work well for these compounds.